I. Field of the Invention
The present invention is directed to a yoke apparatus for use with a rack-and-pinion steering system.
II. Description of the Prior Art
Automotive steering systems typically include a housing having a rack driven by a pinion gear. Rotation of a steering wheel turns the pinion gear. The pinion gear meshes with a plurality of teeth formed on the rack to drive the rack in one of two reciprocal directions. The rack in turn is connected to a pair of dirigible wheels. In addition, many automobile steering systems comprise a rotary control valve which is operable to supply pressurized fluid to move a double-acting hydraulic cylinder or actuator to assist translation of the rack.
In order to keep the teeth of the pinion gear and the rack in engagement, such steering systems employ a yoke apparatus. The yoke apparatus includes a bearing member which is biased to force the rack towards the pinion gear. The bearing member has a pair of spaced apart bearing surfaces which slidingly contact the surface of the rack opposite the teeth of the rack. The bearing member is slidingly mounted in a bore which is formed in a nominally orthogonal manner with reference to the rack's intended position. This results in a nominal alignment of the bearing surfaces along an axis which extends coaxially with the axis of translation of the rack. A spring is mounted in the bore to force the yoke assembly against the rack and bias the bearing surfaces in order to force the teeth of the rack against the teeth of the pinion gear. Thus, the yoke apparatus operates to nominally guide the rack along the axis of translation and hold the teeth of the rack and the teeth of the pinion gear in mesh during the application of torque to the pinion gear.
In practice, it is not possible to maintain the axis of translation of the rack orthogonal to the axis of the bore. This is because of the tolerances involved in forming the bore, rack, and pinion gear. Accordingly, it has been found that the axis of translation of the rack may be angled with respect to the axis of the bearing surfaces of the bearing member, and may even undulate as a function of rotational motion of the pinion gear. When so misaligned, one end of each of the support surfaces engage the rack while opposite ends of the support surfaces are spaced away from the rack. As a further result, the bearing member itself may suffer angular misalignment within the bore and jam. In fact, such yoke assemblies may be said to be over constrained or to be of non-kinematic design.
For the above reasons, the rack is often held from smooth movement in one, or both, directions of travel. This is particularly so when the rack travels in a direction from the contacting ends towards the non-contacting ends of the support surface. The edges resist movement of the rack and the rack tends to hesitate and jerk in its movement. However, movement of the rack in an opposite direction tends to produce a smoother, less resistant movement. Frequently, the discontinuous or halting movement of the rack will be tactilly sensed by the driver.
The spring is located in an adjuster plug which is threadably inserted in the outer portion of the bore. During the assembly of the yoke apparatus, the adjuster plug is rotatably driven into contact with the bearing member with a nominal torque value of perhaps 50 in.lbs. to provide a rotational position reference. Because of the above noted tolerances involved in forming the bore, rack and pinion gear, there results a soft contact between the adjuster plug and the bearing member, and thus an imprecise rotational position reference. For this reason, the adjuster plug must then be backed off by an angle of about 30.degree. in order to ensure interference free operation in the manner described above. This results in an indefinite stop position of the bearing member should a torque level be applied that is sufficient to overcome the spring bias.
In operation, rack and pinion assemblies are often subject to a condition known as "rattle". Rattle most often occurs when the dirigible wheels are subject to dissimilar impacts such as when crossing obliquely disposed railroad tracks or similar road surface discontinuities. It has been found that lateral and rotational motions and resulting impacts within the housing by the yoke apparatus elements are the most significant cause of rattle. This occurs because of the helically formed teeth on the pinion gear. As the pinion gear is caused to rotate by axial thrust loads imposed upon the rack with resulting axial motion thereof, the rack is driven laterally by the helically formed teeth of the pinion gear. In addition, physical separation of the rack and pinion gear interface as a consequence of the heavy shock loads results in impact loads being impressed upon the adjuster plug as another form of rattle. In any case, rattle is typically treated by tightening up various clearances or adding a circumferential elastomeric guide element around the skirt of the yoke, and as a last resort, by significantly increasing the biasing spring force.